1. Field of the Invention
The present invention relates to a combustion apparatus for supplying a high-temperature combustion gas to a suitable machine or plant, for example such as gas turbines, heating furnaces, boilers and the like, and more particularly to a combustion apparatus for supplying a high-temperature combustion gas to multistage combustion-gas utilization systems connected to each other in series, in which combustion apparatus an air/fuel mixture gas is brought into contact with a number of high-temperature heating surfaces to attain a complete combustion thereof so as to produce the high-temperature combustion gas which is substantially free from oxygen.
2. Description of the Prior Art
The combustion gas produced in this type of combustion apparatus is preferably produced through a complete combustion so as to be free from any of pollutants such as nitrogen oxides (NO.sub.x), unburned hydrocarbons and carbon monoxide, and to be sufficiently low in its residual oxygen content.
Heretofore, in this type of combustion apparatus, the following three combustion apparatuses have been proposed:
(1) A combustion apparatus in which: fuel is mixed with air to prepare an air/fuel mixture gas an air/fuel ratio of which is within a flammable limit range; an electric spark from an igniter plug of the apparatus initiates the combustion of the mixture gas; and thereby the air/fuel mixture gas is continuously burned;
(2) A combustion apparatus disclosed in Japanese Patent Laid-Open No. 62-33213 filed by inventors of the present invention, in which combustion apparatus: fuel is mixed with a preheated air to prepare a preheated air/fuel mixture gas which reacts with catalysts to attain its catalytic combustion; and
(3) A combustion apparatus disclosed in Japanese Patent Laid-Open No. 62-116808 filed by the inventors of the present invention, in which combustion apparatus: an outer peripheral surface of a heating tube of a main combustion unit is constantly heated to temperatures of ignition points of fuels by means of an auxiliary combustion unit; and the fuels are brought into contact with the thus heated heating tube of the main combustion unit so as to be burned continuously.
Of the conventional combustion apparatuses described in the above items (1) to (3):
The combustion apparatus described in the item (1) suffers from the fact that the combustion gas is partially kept at a high temperature in order to keep the flame alight, which high temperature makes it impossible to prevent nitrogen oxides (NO.sub.X) from occurring in the combustion gas. In this combustion apparatus, since the time taken for fuel to mix with air is very short and the time taken for the thus prepared air/fuel mixture gas to be burned is also very short, the combustion gas produced in the combustion apparatus of the item (1) suffers from pollutants such as unburned hydrocarbons and the like together with a considerable amount of residual oxygen therein;
The combustion apparatus described in the item (2) enables the air/fuel mixture gas to burst into flame at a relatively low temperature. However, when the mixture gas is burned at a temperature of more than 1300.degree. C., such high-temperature combustion of the mixture gas shortens the service life of the catalysts employed in the combustion apparatus, which makes it impossible to operate the combustion apparatus over an extended period of time. In addition, in this combustion apparatus, it is impossible to use sulfur-containing fuels because the sulfur has poisoning effects on the catalysts; and
The combustion apparatus described in the item (3) suffers from the fact that the auxiliary combustion unit is also constantly kept in operation together with the main combustion unit, which disturbs the operator of the apparatus in control of both units. In addition, in this combustion apparatus, it is not possible to substantially completely prevent nitrogen oxides (NO.sub.X) from occurring in the combustion gas.
In an air/fuel mixing unit employed in this type of combustion apparatus, an oxygen-containing gas such as air is mixed with fuel at a predetermined ratio to prepare an air/fuel mixture gas which is supplied under a predetermined pressure to the combustion unit of the apparatus. The air/fuel mixing unit is provided with a venturi tube having a venturi throat portion in which a fuel discharge nozzle is provided. As a preheated air flows through the throat portion of the venturi tube of the air/fuel mixing unit, a partial vacuum is produced at the venturi throat portion. This vacuum then causes the fuel discharge nozzle to deliver a spray of fuel into the passing preheated air stream in the venturi throat portion.
In the above combustion apparatus, in case that the combustion is conducted at a temperature of from 1200.degree. to 1400.degree. C., the fuel is mixed with the oxygen-containing preheated gas or air in the air/fuel mixing unit, which air has been preheated to a temperature of from 500.degree. to 900.degree. C. In this case, the fuel discharge nozzle provided in the throat portion of the venturi tube is also heated to a temperature of from 500.degree. to 900.degree. C. together with a fuel feed pipe connected with the fuel discharge nozzle. As a result, the fuel is often pyrolyzed or thermally decomposed to precipitate carbon particles in the fuel discharge nozzle and the fuel feed pipe. The thus precipitated carbon particles often clog these nozzle and pipe.
Particularly, in case that the combustion apparatus for supplying a high-temperature combustion gas (the temperature of which is about 1400.degree. C.) to each of a plurality of boilers connected to each other in series is provided in a front portion of each of the boilers: the combustion gas discharged from the first one of the combustion apparatuses is supplied to the first one of the boilers; the combustion gas having passed through such first boiler is then supplied to the second one of the combustion apparatuses; the combustion gas having passed through such second combustion apparatus is supplied to the second one of the boilers; and operations similar to the above are sequentially conducted through the remaining boilers and combustion apparatuses until the combustion gas passes through the last one of the boilers; whereby multistage utilization of the combustion gas is realized. In this case, the combustion gas is supplied at a temperature of about 1400.degree. C. to each of the boilers, while discharged at a temperature of about 700.degree. C. from each of the boilers after it passes through each of the boilers. The thus discharged combustion gas is supplied to an air/fuel mixing unit of a subsequent combustion apparatus so as to be mixed with fuel and burned to be heated again to a temperature of about 1400.degree. C. At this time, since the venturi tube of the air/fuel mixing unit has been heated to a considerably high temperature in a portion in the vicinity of the fuel discharge nozzle, the fuel in the nozzle is often pyrolyzed or thermally decomposed to precipitate carbon particles which disadvantageously clog the fuel discharge nozzle in the venturi tube of the air/fuel mixing unit.